Basic FAQ: Introduction to Battle Positions

A battle position, known as an indication form in the Japanese version, is a status in which a Monster Card is placed to determine how it behaves in battle. The two battle positions are Attack Position and Defense Position.
Attack Position monsters are placed vertically on the field, while Defense Position monsters are placed horizontally.

Attack Position monsters are able to declare attacks. If two Attack Position monsters battle, the monster with lower ATK is destroyed and its controller loses Life Points equal to the difference in ATK. If an Attack Position monster battles a Defense Position monster with a DEF greater than the attacking monster's ATK, neither monster is destroyed. However, the attacking player loses Life Points equal to the difference between the ATK and DEF. If a Defense Position monster is attacked, its controller does not lose Life Points.

Attack Position is a Battle position in which a Monster Card is face-up in a Vertical Position. In this position, the ATK value is used to represent this card, except where a card effect requires a specific examination of the DEF value.

Attack Position monsters should always be face-up. The only time when a face-down Attack Position Monster Card is acceptable is when it is placed that way by the effect of "Darkness Approaches"; there are no other cards involving face-down Attack Position. However, most players tend to ignore this rule because they refer to face-down Defense Position Monsters as face-down Monsters.

Example


Defense Position is a Battle position in which a Monster Card can be either face-up or face-down, in a Horizontal Position. In this position, the DEF value is used to represent this card, except where a card effect requires a specific examination of the ATK value.
In Defense Position, a Monster Card is always in a Horizontal Position. When a monster is Set, it is played in face-down Defense Position. Monsters cannot be Normal Summoned in face-up Defense Position, except by a card effect that allows them to be summoned that way (such as "Light of Intervention"). During a battle involving a Defense Position monster, the DEF value of that monster is used to calculate Battle Damage, rather than the ATK value.

Usually, Defense Position monsters cannot attack, but some monsters such as "Elemental HERO Rampart Blaster" and "Total Defense Shogun" have effects that allow them to attack while in Defense Position.

Also, the controller of a Defense Position monster takes no damage if it is destroyed in battle, even if the attacker has higher ATK than his monster's DEF. Exceptions do exist, as some cards grant the piercing ability, which lets a monster inflict damage in this way. Examples include monsters such as "Spear Dragon" and "Gaia Soul the Combustible Collective", as well as Spell and Trap Cards such as "Big Bang Shot" and "Dragon's Rage". If the attacker has lower ATK than the DEF of the attack target, the attacker takes Battle Damage, but neither monster is destroyed. If the attacker's ATK is equal to the target's DEF, neither player takes Battle Damage and neither monster is destroyed.

Example
http://images1.wikia.nocookie.net/__cb20081215225533/yugioh/images/1/15/BlueEyesWhiteDragon-JMP-NA-UR-UE-Defense.png

A monster's battle position can be changed in one of two ways: manually or by card effect.

A manual battle position change can only be performed when the game state is open during your Main Phase. It is done by changing the battle position of 1 monster you control.

Each monster's battle position can only be changed manually once per turn.

You cannot change the battle position of a monster manually during the same turn it was Summoned, Normal Set, or declared an attack (even if the attack was negated or canceled due to a replay).

You cannot change monsters to face-down Attack Position whether manually or by a card effect. Face-down Defense Position monsters are always flipped when changed to Attack Position. This change is called a Flip Summon when it's done manually.